Low stress magnet interface for a force rebalance accelerometer

ABSTRACT

A force rebalance accelerometer includes a proof mass suspended by one or more flexures between stationary mounted upper and lower excitation rings. Pick-off capacitance plates formed on opposing sides of the proof mass form capacitance elements whose capacitance varies in response to displacement of the proof mass to provide a displacement signal. The displacement signal is applied to one or more electromagnets, used to force the proof mass back to a null or at-rest position. The drive current applied to the electromagnets thus represents the force or acceleration applied to the accelerometer. The electromagnets include a magnet and a pole piece which forms a magnetic return path. In order to relieve stresses due to thermal expansion, the magnet is spaced apart from the pole piece to enable the bonding area to be constrained to a minimum which, in turn, reduces the overall stress on the accelerometer.

This application is a divisional of U.S. Ser. No. 08/184,527, filed Jan.21, 1994, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to an accelerometer and, moreparticularly, to a stress isolation groove for reducing mechanicalstress at the magnet interface of a force rebalance accelerometer whichincludes a proof mass suspended between one or more magnet assemblies.

2. Description of the Prior Art

Force rebalance accelerometers which include a proof mass suspendedbetween one or more magnet assemblies are generally known in the art.Examples of such accelerometers are disclosed in U.S. Pat. Nos.4,182,197; 4,250,757; 4,394,405; 4,399,700; 4,400,979; 4,441,366;4,555,944; 4,555,945; 4,592,234; 4,620,442; 4,697,455; 4,726,228;4,932,258; 4,944,184; 5,024,089; 5,085,079; 5,090,243; 5,097,172;5,111,694; 5,182,949; 5,203,210; 5,212,984; and 5,220,831, all hereinincorporated by reference. Such force rebalance accelerometers normallyinclude a proof mass, known to be formed from amorphous quartz,suspended by one or more flexures to enable the proof mass to deflect inresponse to forces or accelerations along a sensitive axis, generallyperpendicular to the plane of the proof mass. At rest, the proof mass isnormally suspended equidistantly between upper and lower excitationrings Electrically conductive material forming pick-off capacitanceplates, is disposed on opposing sides of the proof mass to formcapacitive elements with the excitation rings. An acceleration or forceapplied along the sensitive axis causes the proof mass to deflect eitherupwardly or downwardly which causes the distance between the pick-offcapacitance plates and the upper and lower excitation rings to vary.This change in the distance between the pick-off capacitance plates andthe upper and lower excitation rings causes a change in the capacitanceof the capacitive elements. The difference in the capacitances of thecapacitive elements is thus representative of the displacement of theproof mass along the sensitive axis. This displacement signal is appliedto a servo system that includes one or more electromagnets whichfunction to return the proof mass to its null or at-rest position. Themagnitude of the drive currents applied to the electromagnets, in turn,is representative of the acceleration or force along the sensitive axis.

The electromagnets are known to include a magnet formed from, forexample, alnico, normally bonded to an excitation ring formed from amaterial having relatively high permeability, such as Invar, to form amagnetic return path. The materials used for the magnet and theexcitation ring will have different coefficients of thermal expansion,since the materials are different. As such, the interface definedbetween the magnet and the excitation ring will be subject to stress asa function of temperature. Such stress over a period of time and/ortemperature degrades the performance of the accelerometer.

In order to resolve this problem, compliant epoxies have been used tobond the magnet to the excitation ring. However, such compliant epoxiesdegrade the long term stability of the accelerometer.

SUMMARY OF THE INVENTION

It is an object of the present invention to solve various problemsassociated with the prior art.

It is yet another object of the present invention to provide a forcerebalance accelerometer which minimizes stress of the accelerometer dueto temperature expansion without the use of a compliant epoxy.

It is yet another object of the present invention to provide a forcerebalance accelerometer which provides relatively stable output overtemperature and over a relatively long period of time.

Briefly, the present invention relates to a force rebalanceaccelerometer which includes a proof mass suspended by one or moreflexures between stationary mounted upper and lower excitation rings.Pick-off capacitance plates formed on opposing sides of the proof massare used to form capacitance elements whose capacitance varies inresponse to displacement of the proof mass to provide a displacementsignal. The displacement signal is applied to one or moreelectromagnets, used to force the proof mass back to a null or at-restposition. The drive current applied to the electromagnets thusrepresents the force or acceleration applied to the accelerometer. Theelectromagnets include a magnet, rigidly secured to an excitation ringwhich forms a magnetic return path. In order no relieve stresses due tothermal expansion, the magnet is slightly elevated and the bonding areais constrained to a minimum. By relieving the stress at the magnetinterface, the performance of the accelerometer in accordance with thepresent invention will be relatively stable over time.

BRIEF DESCRIPTION OF THE DRAWING

These and other objects of the present invention will be readilyunderstood with reference to the following detailed description andattached drawing, wherein:

FIG. 1 is an exploded perspective view of a known force rebalanceaccelerometer;

FIG. 2 is a simplified cross-sectional view of a known force rebalanceaccelerometer; and

FIG. 3 is a partial cross-sectional view of a magnet assembly inaccordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates a known force rebalance accelerometer, generallyidentified with the reference numeral 20. The force rebalanceaccelerometer includes one or more magnet assemblies 22 and a proof massassembly 24. The proof mass assembly 24 includes a mounting ring 26 anda generally paddle-shaped proof mass 28. The proof mass 28 is suspendedrelative to the mounting ring 26 by way of a pair of flexures 30 toenable the proof mass 28 to rotate relative to the mounting ring 26.Cylindrically shaped bobbins 32 and 34 are formed on opposing surfacesof the proof mass 28. The bobbins 32 are used to carry torquer coils 36and 38. Conductive material 40 is deposited on the opposing surfaces ofthe proof mass 28 to form pick-off capacitance plates.

The magnet assemblies 22 include a permanent magnet 42 and a generallycylindrical excitation ring or flux concentrator 44. The excitation ring44 is configured to have a generally C-shaped cross section. Thematerial for the excitation ring 44 is selected to have relatively highpermeability, such as Invar, to form a magnetic return path. Inwardlyfacing surfaces 46 on the excitation rings 44 form in combination withthe conductive material 40 on the opposing sides of the proof mass 28form variable capacitance elements PO1 and P02 as shown in FIGS. 1 and2.

Referring to FIG. 2, the proof mass 28 is shown at an at-rest or nullposition. In this position, the distance between the surfaces 46 of theupper and lower excitations rings 44 and the pick-off capacitance plates40 are equal. Since capacitance is a function of the distance betweenthe plates, the capacitance values of the capacitors PO1 and PO2 areequal during this condition.

In response to an acceleration or force along a sensitive axis S,generally perpendicular to the plane of the proof mass 28, the proofmass 28 moves toward one or the other of the excitation rings 44. Thisdisplacement of the proof mass 28 changes the respective distancesbetween the surfaces on the pick-off capacitance plates 46 formed on theopposing sides of the proof mass 28 relative to the upper and lowerexcitation rings 44. This change in the distance results in a change inthe capacitance of the capacitive elements PO1 and PO2. Circuitry formeasuring this change in capacitance is disclosed in U.S. Pat. No.4,634,965 and co-pending application Ser. No. 08/151,417, filed on Nov.12, 1993 by Paul W. Rashford and entitled "IMPROVEMENT OF CHARGEBALANCING CIRCUIT" and incorporated herein by reference.

The difference in the values of the capacitances PO1 and PO2 isrepresentative of the displacement of the proof mass 28 either upwardlyor downwardly along the sensitive axis S. This displacement signal isapplied to a servo system which includes the magnet assemblies 22 andthe torquer coils 36 which form electromagnets to return the proof mass28 to its null position. The magnitude of the drive current to theelectromagnets is a measure of the acceleration of the proof mass 28along the sensitive axis S.

The magnet assembly 60 in accordance with the present invention,generally identified with the reference numeral 60 (FIG. 3), solvesthese problems. The magnet assembly 60 includes an excitation ring 61, amagnet 42 and a pole piece 62. The excitation ring 61 is formed in agenerally cylindrical shape with a C cross section. The magnet 42 havingopposing bonding surfaces 63 is centrally secured to a base portion 64of the excitation ring 61. As mentioned above, known force rebalanceaccelerometers include a magnet assembly which includes an excitationring bonded to the entire bonding surface of the magnet. Additionally, apole piece may be bonded to an opposing surface of the magnet. Due tothe difference in materials used for the magnet, the pole piece and theexcitation ring, the differing rates of thermal expansion cause stressat the magnet to excitation ring interface and the magnet to pole pieceinterface. This stress produces distortion in the excitation ring andthe pole piece which degrades performance. Since the magnet is normallybonded to the excitation ring with an epoxy, such stress also weakensthe bonding over time and, as such, degrades the performance of theaccelerometer.

The magnet assembly 60 in accordance with the present invention isformed such that the magnet 42 is spaced apart from the base portion 64of the excitation ring 61 by a relatively small gap 65. In order tominimize the stress due to thermal expansion, the bonding material 66 isconstrained to a minimum as shown in FIG. 3 forming a pedestal to covera relatively small portion of the bottom surface area 63 of the magnet42.

The pole piece 62 may be bonded to the other pole face or bondingsurface 63 of the magnet 42 to form the magnet assembly 60. In order toreduce stress due to temperature at this interface, the pole piece 62 isbonded only to a small portion of the bonding surfaces 63 of the magnet42.

Since the bonding material 66 is non-magnetic, the addition of the airgap 65 has little effect on the magnetic circuit. In addition, byminimizing the bonding area, the overall stress is significantlyreduced, thus providing an accelerometer with a relatively stable outputover time.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. Thus, it is to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described above.

What is claimed and desired to be secured by Letters Patent of theUnited States is:
 1. A force rebalance accelerometer, comprising:a proofmass; a mounting ring; a pair of flexures for flexibility connecting theproof mass to the mounting ring; means for returning the proof mass to anull position, said returning means including a permanent magnet havinga first and a second opposing bonding surfaces and at least oneexcitation ring said first bonding surface of said magnet spacedadjacent to said excitation ring; and means for bonding said firstbonding surface of said magnet to said excitation ring, said bondingmeans only bonding a predetermined portion of said first bonding surfaceof said magnet thereby forming a pedestal and wherein a gap existsbetween a remaining portion of said first bonding surface and saidexcitation ring.
 2. A force rebalance accelerometer as recited in claim1, further including a pole piece bonded to said second bonding surfaceof said magnet.
 3. A force rebalance accelerometer as recited in claim1, further including a generally circular pole piece and means forbonding said pole piece to one of said first or second bonding surfacesof said magnet, said bonding means only bonding a predetermined portionof said bonding surface of said magnet thereby forming a pedestal.